1. Field of the Invention
This invention relates to an optical node for coupling the transmission lines of an optical information system to each end and an optical information system using the same.
2. Related Background Art
In a bus type optical information system, as shown in FIG. 1 of the accompanying drawings, optical nodes 102 are provided in a portion of an optical fiber bus line 101 and an end instrument 103 having the transmitting and receiving function is connected to each of the optical nodes, whereby the information transmission between the ends is effected through the bus line. As optical nodes used in such a bus system, there have been proposed various types such as (1) the light receiving and emitting type and (2) the passive light branching-off type.
The light receiving emitting type mentioned under item (1) above is a method in which after a light signal is converted into an electrical signal, a light signal is again transmitted, and a light emitting and receiving device is inserted on a bus line. This method, however, has suffered from a problem such as the time delay of the signal due to the delay of the reproduction time and in addition, a problem that the reproduction of a wavelength multiplexing signal is cumbersome.
On the other hand, the passive light branching-off type mentioned under item (2) above is such that as shown in FIG. 2 of the accompanying drawings, a T-shaped branching-off coupler 104 is installed on a bus line 101 and the introduction of a signal on the bus line and the transmission of the signal to the bus line are effected to thereby realize the information transmission between optical nodes. A receiving unit 106 and a transmitting unit 107 are connected under the T-shaped branching-off coupler 104 through a Y-shaped branching-off coupler 105. However, in such a passive type optical branching-off node, the attenuation of light power due to the branch-off loss is remarkable, and this has led to the basic problem that the number of optical nodes installed is limited.
Also, the dynamic range of the signal on the bus line becomes great due to the branch-off coupling loss, and this has led to the problem that the burden for the performance of a photodetector becomes great.
In order to make up for such branch-off coupling loss, there is a method of inserting an optical amplifier 108 onto the bus line and directly amplifying the light signal, but this imparts a new coupling loss because of the increase in optical fiber coupling regions, and this in turn has led to the problem that a high amplification degree is required of the optical amplifier 108.
On the other hand, as similar examples in which an optical amplifier and a branching-off coupler are combined together, there have been proposed several, examples in which a compound type laser resonator is constructed.
For example, in I. H. A. Fattah et al., "Semiconductor interterometric laser", Appl. Phys. Lett. 41, 2, pp 112-114 (July, 1982), there is described an interference type laser including a y branch-off as shown in FIG. 3 of the accompanying drawings. Also, in J. Salzman et. al., "Cross coupled cavity semiconductor laser", Appl. Phys. Lett. 52, 10, pp. 767-769 (March, 1988), there is described an interference type laser including x branch-off as shown in FIGS. 4A and 4B of the accompanying drawings. Here, R.sub.1 -R.sub.4 designate resonating surfaces, and L.sub.1 -L.sub.4 indicate the lengths of resonators.
Further, in Japanese Laid-Open Patent Application No. 62-145225, it is proposed to compensate for branch-off loss by the use of such a branching-off type laser.
However, it has not at all been conceived to utilize these lasers as the node of an optical information system.